While significant study has been directed at understanding the genetic basis of cancer, there is growing vidence that epigenetic mechanisms also play a significant role. Epigenetic mechanisms, such as hromatin modifications and DNA methylation, are stable, long-term (typically heritable) changes in the ranscriptional potential of a cell that are independent of changes in the underlying genomic sequence. These epigenetic modifications can reveal the transcriptional history and key control mechanisms for protein-coding and miRNA genes. T-cell acute lymphoblastic leukemia (T-ALL) is a neoplastic disorder of lymphoblasts arising in the T-cell lineage. The major subtype of human T-ALL can be defined by cytogenetic abnormalities and differentiation arrest at different stages of T-cell development. NOTCH1 serves as a unifying target in this model, as activating NOTCHI mutations have now been found in all of the most common subtypes of T-ALL and in more than 50% of all pediatric T-ALL cases. Our central hypothesis is that comparing the genome-wide epigenetic signatures of T-ALL cells to normal T cell precursors will lead to substantial new insights, including the identification of genes that are differentially regulated, as well as putative markers that might lead to early diagnosis and/or improved monitoring of the progress of tumor therapies. We propose to determine the dynamic changes to cell potential using high-quality epigenetic signatures of chromatin modifications and transcriptional potential during mammalian T cell development and leukemogenesis using a NOTCH-induced mouse model (AIM 1) to determine the epigenetic mechanisms involved in progression of the T-cell leukemias. Information gathered using the mouse models will complement epigenetic state of specific subtypes of human T-ALL with defined genetic mutations (TAL1 pos, +/- NOTCH mutations) (AIM 2). The long-range goal of this proposal is to identify the transcriptional history and key epigenetic control mechanisms genome-wide for all protein-coding and miRNA genes during mammalian T cell development and T cell leukemogenesis and thus provide a critical signature of leukemic identity.